A wide variety of different types of electronic control circuitry are known. These include, for example, boiler controls for hot water heating systems. A typical hot water boiler control utilizes a switch responsive to an output signal from a boiler water temperature sensor to control burner fuel supply and ignition. The boiler water temperature sensor may comprise a heat sensitive capillary tube inserted into an enclosed well in the hot water boiler. The hot water heating system further comprises a room or area thermostat that is used to control a circulator. The circulator is configured to circulate hot water from the boiler through hot water radiators associated with the room or area in order to maintain a desired room or area temperature.
In these and other typical boiler controls, temperature settings of the switch that controls burner fuel supply and ignition are confined to a relatively narrow range selected to accommodate the coldest outside temperature that is likely to be experienced. More particularly, the temperature settings generally must be high enough for the coldest outdoor conditions likely to be experienced so that the hot water radiators will be capable of providing a comfortable room or area temperature under those conditions. However, at higher boiler temperatures there is greater heat loss, greater fuel usage and greater greenhouse emissions. Since outside temperatures vary throughout the year, it is advantageous and more efficient for boiler temperatures to be reduced as outside temperatures increase. Comfortable room or area temperatures can be maintained using significantly reduced boiler temperatures in the presence of higher outdoor temperatures. This is especially important during spring, summer and fall times when the boiler is not used for heating but merely to provide hot water via its internal hot water heat exchangers. Under these conditions, high boiler temperatures are not needed, are particularly inefficient and unnecessarily produce excessive greenhouse emissions.
A number of different approaches attempt to address the inefficiencies associated with maintaining unduly high boiler temperatures. For example, some of these approaches involve measuring the outside temperature and reducing boiler temperature as the outside temperature increases. Other approaches involve comparing the measured temperature of the water leaving the boiler and going to the hot water radiators to the measured temperature of the water returning to the boiler from the hot water radiators, and reducing boiler temperature as the difference between the measured temperatures decreases.
Although these techniques can be effective, they suffer from a number of significant drawbacks. For example, some techniques require the addition of outdoor temperature sensors or the utilization of complex digital data processors and associated software. This adds excessive cost to the boiler control and to its installation. This also increases the number of boiler control failure modes. For example, single point failures of critical components can cause a temperature runaway condition in the boiler.